Bridging connection for program circuits



March 7, 1933. H. s. HAMILTON ET AL BRIDGING CONNECTION FOR PROGRAM CIRCUITS Filed Oct. 27, 1931 ATTORNEY Patented Mar. 7, 1933 HAROLD S. HAMILTON, 0F WHITE PLAINS, NEW YORK, AND GEORGE CRISSON, OF EAST' PAT ORANGE, NEW JERSEY, ASSIGNORS TO AMERICAN TELEPHONE AND TELEGRAIH COMPANY, A CORPORATION OF NEW YORK BRIDGING- CONNECTION FOR` PROGRAM CIRCUITS Application filed October 27, 1931.

This invention relates to transmission circuits, and more particularly toarrangements whereby a main transmission line may have a number of branch or auxiliary lines bridged thereto so as to receive transmission from the main line in multiple.

In program transmission itis frequently desirable to have a main program line connectedso that the program transmitted thereover may be transmitted into each of a number of branch lines. In accordance with the present invention the branch lines are connected in bridged relation with respect to the main line so as to receive transmission therefrom in multiple with respect to each other.

In order that any branch line may be discon-l nected from the network without affecting the existing impedance relations the circuit is arranged so that when any branch line is disconnected it will be replaced by its equivalent impedance.

rlhe bridging arrangement is also designed so that the impedance looking into the bridging arrangement from the connection lead-v ing to any line will be equal to that of the line itself. This is accomplished by inserting series impedances in each branch of the bridging arrangement ahead of the branch line. These impedances also serve the purpose of preventing an accidental short across a branch line from disabling other branch lines.

rlhe bridging network above described in* troduces a certain amount of transmission loss, and in accordance with the present invention this is made up by inserting inthe main line an amplifier having its gain adjusted to compensate for the loss in the bridging network. rIhis amplifier is preferably connected to the network by a transformer having such a ratio as to step up the impedancer looking into the bridging vnetwork to the impedance of the output circuit of the amplifier.

The invention Will now be more fully un- Serial N0. 571,450.

derstood from the following description when read in connection with the accompanying drawing, in which Figure l shows a circuit arrangement embodying the present invention, and Figs. 2, 3, 4 and 5 are equivalent diagrams showing how the impedance looking into each'branch of the bridging arrangement is made equal to that of the branch line itself.

Referring to Fig. 1, ML designates a main p various branch lines Ll to LG, inclusive to the y circuit. The bridging connections terminate injacks .Il to J6, inclusive, in which plugs P1 to P6, inclusive, of the various branch lines may be inserted to complete the connections to the branch lines. In the idle condition of the jacks, resistances X1, X2, X3, etc., are connected across the jack springs. These resistances are made equal to the impedance looking into each branch line, so that the connection or disconnection of any branch line does not affect the impedance looking into the bridging circuit l0 from the output of the amplifier A. Each of the branch lines L1, L2, etc., is provided with an amplifier indicated at A1, A2, etc.

It is, of course, necessary in order to avoid variations of the gain-frequency curves of amplifiers A, etc. when their gains are changed, that the impedance looking from a branch line into any bridging circuit such as B1 and B2 towards the .amplifier A should be equal to the impedance looking into the branch line itself. Accordingly, resistances such as R1 and R', are inserted in the bridging circuit B1 ahead of the jack J1, and similar resist-ences are inserted in the other bridging circuits. These resistances also serve the useful purpose of preventing an accidental shorting of any of the branch lines from materially affecting the transmission in the other branch lines.

The bridging network above described, introduces a certain amount of loss in transmission from the main line ML to the input of the amplifiers such as A1 or A2 in the branch lines, and the gain of the repeater A is adjusted so that it will inst compensate for the loss so introduced. Tn order that there may 'be no impedance irregularity7 between the output of the repeater A and the bridging circuit, the impedance looking into the bridging circuit 10 which is that looking into the several bridging` circuits in parallel, is stepped un by means of the transformer T to the impedance of the output circuit of the amplifier A.

If we assume that each branch line has an impedance of 600 ohms looking into its ampliiier such as A1. each of the series resistances in each bridging circuit shouid have a value of 250 ohms ordei` that the impedance looking into each bridging circuit will be equal to that loking` into the branch line amplifier. If no series resistances such as R1, RQ, etc., were provided, the impedance looking into the circuit 10 towards the bridging circuit would be that of six circuits in parallel, each havingl G00 ohms impedance, or 100 ohms. lf then the impedance looking into the transformer T is made equal to the impedance in the other direction, the impedance looking into the output of the amplifier A would also be 100 ohms. The impedance looking towards the amplifier from any bridging circuit such as B1, would then be equivalent to an impedance of 100 ohms looking into the amplifier in parallel with the impedance of five 000 ohm circuits connected in parallel with each other: that is, a 100 ohm impedance in parallel with an impedance of 120 ohms. Consequently. there would be a very serious impedance irregularity in passing from the amplifier A through the bridging circuit to the 600 ohm branch line.

Tf, however, an impedance` of 250 ohms is inserted in each side of each bridging circuit, this impedance irregularity wil... 4be eliminated. The impedance looking into each branch, taken alone, will now be 1100 ohms, and the impedance as seen from the circuit 10 will be that of 1100 ohm circuits in parallel, or 183% ohms. Assuming that theV transformer T has s ch ratio to step the output impedance of the amplifier A down to 1831/; ohms, the impedance looking from the circuit 10 into the transformer T will then also be 1831/3 ohms. The equivalent impedance as seen from any bridging circuit as, for example, the bridging circuit B1, will then he as indicated in 2 which shows 5 parallel impedances of 1100 ohms each (these being the impedances of the five branch circuits BZ to B5, inclusive) in parallel with the impedance looking into the transformer T of 1331/3 ohms, and in series with this combination the 250 ohm impedances included in the branch circuit itself. Since the five 1100 ohm impedances in parallel are equivalent to a single impedance of 220 ohms, the circuit reduces to the simplied form shown in Fie'. 3. But, 220 ohms in parallel with 1831/7, ohms is equivalent to an impedance of 100 ohms so that we have, looking into any bridging circuit, 250 ohms in series with 100 ohms in series with a. second resistance of 250 ohms, or a total of 600 ohms as shown in Fig. 4;.

As already stated, the impedance looking from the circuit 10 into the bridging` network will be that of the six bridging circuits in multiple. As each bridging circuit has an impedance of 600 ohms which is seen through a series impedance of 500 ohms (250 ohms in each sido) the actual impedance looking into each bridging circuit is 1100 ohms. Six 1100 ohm impedances in parallel are equivalent to an impedance of 183% ohms so that this is the value of the impedance looking from the circuit 10 into the network. The impedance looking in the opposite direction from the transformer T should have substantially the same value. As the output circuit of the amplifier has an impedance of approXimate ly 7000 ohms, by making the ratio of the transformer T I1000 to 135, the impedance looking into the transformer will substantially match that looking' into the network.

The impedance looking into the main line ML would also normally be 600 ohms, and the connection looking into the amplifier A should therefore be 600 ohms. As the grid circuit of the amplier is practically infinite the impedance looking through the transformer T will also be practically infinite, and the taps on the potentiometer P are, from an impedance standpoint, equivalent to having the primary circuit of the transformer T open circuited. Since the potentiometer P has an impedance of 800 ohms, and this impedance is shunted by a resistance of 2400 ohms, the impedance looking into the combination towards the repeater will be 600 ohms, as is required.

The values of the resistances R1, R1', etc.Il may be computed in the following manner. Referring to Fig. 5 which shows the equiva lent of the impedance looking to the left from any branch circuit, we have two unlrnown series impedances X whose value is to he determined, and beyond these series impedances is a shunt impedance Z2 which is ZX-I- 600 5 The impedance Z1 of Fig. 1 is the impedance looking into the transformer T and should be equal to the impedance lookin to the right into the siX bridging circuits. onsequently, the impedance Z1 will be equal to the impedances of six bridging circuits in parallel or Now, since the impedance looking into the circuit of Fig. 5 is to beequal to 600 ohms (the impedance looking into the branch line amplifier) we have by setting down the equation of Fig. 5,

from which we get X=250- If each of the resistances R1 and R1', etc., are given this value the impedance of the bridging network, as seen from any bridging circuit will be 600 ohms, and the impedance of the network as seen from the output side of the transformer T will be equal to the impedance looking into the output circuit of the amplifier A through the transformer T, so that no impedance irregularities will occur. When any branch line such as L3, for eX- ample, is disconnected from the bridging circuit by pulling the plug P3 from the jack J3, a corresponding 600 ohm resistance X3 is substituted for the impedance looking into amplifier A3 of the line which is disconnected, and hence, the impedance looking into the network from any of its other terminals is unchanged.

If, through accident, or circuit fault, an open circuit should occur to the right of the series resistances such as R1, R1', etc., the impedance relations of the other branches will not be seriously disturbed. The impedance lbeyond the series resistances of any other bridge terminal will then be that of four bridging circuits instead of five, in parallel with the impedance looking into the transinstead ofl'OO ohms as given dill-Figli.V Consequently, the total impedance looking to the vleft from any bridgingterminal will be about V610 ohms instead of 600 ohms.

lf, instead of an 'open circuit, one of the 'branch' circuits. Yshould be shorted to the rightof ythe 250v ohm resistances, here again the impedances asseen from the other terminals` of the network vwill not be greatly changed. F orexample, the impedance looking into; the networkI from any other bridging terminal will be as shown in Fig. 2, eX- cept that one ofthe fivey 1100 ohm resistances shown will now be a 500 ohm resistance. The "otherfo'ur 1100 OhrnA resistances in parallel are equivalent to an impedance of275 ohms which," in v'} arallel "with ,the impedance of 183% ohms looking into the transformer, will be equivalent to about 110 ohms. This result in parallel with the now defective branch having an impedance of only 500 ohms, is equivalent to about 90 ohms which, taken in series with the two 250 ohm resistances,

gives for the impedance as seen from any branch circuit a total impedance of about 590 ohms instead of the 600 ohms required by theory.

It will be obvious that the general principles herein disclosed may be embodied in many other organizations widely different from those illustrated, without departing from the spirit of the invention as defined in the following claims.

Vhat is claimed is:

l. In a transmission system, a main line including an amplifier, a plurality of branch lines, a bridging circuit interposed between said amplifier and said branch lines, said bridging circuit having multiple bridge connections for said branch lines, impedances.'1

in each bridge connection between the bridging line and the junction point of the bridge connections, saidv impedances having such values that the impedance looking into the bridging circuitffrom any branch line `i will be equal to the impedance of the branch line itself and the impedance looking into the bridging circuit will be less than the impedance of a branch line, and a transformer associating said bridging circuit with the output of said ampliier and equalizing the impedance looking into the amplifier with respect to the bridging circuit.

2. In a transmission system, a main line including an amplifier, a plurality of branch lines, a bridging circuit interposed between said amplifier and said branch lines, said bridging circuit having multiple bridge connections for said branch lines,` a transformer associating said bridging circuit with the output of said amplifier, and impedances in each 65 former `T, which will be about 110 ohms `that the impedance looking into the bridging y1130 circuit from any branch line will be equal to l the impedance of the branch line itself and the impedance looking into the bridging circuit Will be less than the impedance f a w branch line, and the ratio of the transformer being such that the impedance looking into the amplifier from the bridging circuit will be stepped down to the impedance looking into the bridging circuit from the amplifier. Y In testimony whereof, I have signed my name to this specification this lst day of OctOber, 1931..

GEORGE CRISSON. In testimony whereof, I have signed my name to this specification this 22d day of October, 1931.

HAROLD S. HAMILTON. 

